The Pokemon franchise is designed around single-player experiences and PVP multiplayer battles. It's not built for the group party dynamics of a game like Dungeons & Dragons, but the monsters from Pokemon can still be used as enemies for parties to face. To this end, we have statted out Ditto for use in D&D adventures.
Ditto was the original gimmick Pokemon. In Pokemon Red & Blue, it was the only Pokemon (outside of Mew) with the ability to learn Transform, which allowed it to copy the stats (excluding HP) of an enemy Pokemon. This move wasn't as useful as you might think, as it took a turn to use, meaning that the enemy had a free round to wail on Ditto. The fact that Ditto had a horrible base HP stat meant that usually didn't have a chance to respond. Ditto became a bit more useful in later generations, when it gained the Imposter Hidden Ability. Imposter allowed Ditto to transform as soon as it entered the battlefield, so it didn't have to waste a turn using Transform. This didn't help it much, as the low HP score meant that this polymorphing Pokemon was only ever useful for breeding or novelty runs.
Ditto's Stats
There are many reasons why spellcasters take up the magical arts. There are some who want the respect that being able to throw fireballs commands, while others want the power to protect those who cannot protect themselves. To some, there is a more base need for studying the magical arts, and that's the desire to become something else. The Polymorph spell holds a lot of sway in the imagination of the common folk, as it lets them soar through the skies or swim beneath the waves. To some, the desire to see a different face staring back from beyond the mirror is reason enough to become a mage.
The desire to polymorph into another creature is so strong that many enterprising magic-users have made a lot of money from creating items that allow temporary transformations. There are many noblemen and ladies who pay good money for polymorphing treats for their parties. This endeavor is so lucrative that it led to items being mass-produced, with alchemical labs working overtime to produce potions and scrolls that allowed for short-term transformations.
The abundance of transmutation magic at play leads to accidents. No one is quite sure how, but the droplets of potions and sparks can converge into a creature with a purely fluid form. A Ditto is a magical accident, a lifeform created wholly from the errors of mages and sorcerers. Despite this, they are affable little creatures, and they mostly use their incredible powers to bring smiles to faces. There are even some spellcasters who take on a Ditto as a familiar.
A Ditto has the power to wholly transform into a creature it sees, gaining all of its powers in the process. It's said that a Ditto could turn into a mighty Tarrasque if it was able to get close enough to do it. The only problem is that Ditto maintains its docile personality. It also maintains its purple color scheme, so it's not great at deceiving foes. Still, the Ditto can be a potent foe on the battlefield, even for a short while, and it can show incredible bravery when its friends are threatened.